Thomas Stanley, 1st Earl of Derby

Thomas Stanley, 1st Earl of Derby (1435-29 July 1504) was an English nobleman and the stepfather of King Henry VII of England. At the Battle of Bosworth Field in 1485, Stanley's defection from King Richard III's army to Henry's army turned the tide of the battle, and Stanley's troops slew King Richard III. From 1483 to 1504, he served as Lord High Constable of England.

Biography
Thomas Stanley was born in 1435, the son of Thomas Stanley, 1st Baron Stanley. He initially supported the House of Lancaster during the Wars of the Roses, but he refused to fight against his father-in-law, the Earl of Salisbury, at Blore Heath in 1459, and he began to cooperate with the Yorkists in 1460. King Edward IV of England entrusted Stanley with securing North West England, and he served as Steward of the King's Household under Edward. After the death of his first wife, he remarried to Margaret Beaufort, the mother of Henry Tudor, a Lancastrian claimant to the throne. He led several hundred men in the 1475 campaign in France and served in the Duke of Gloucester's 1482 campaign against Scotland, capturing Berwick.

Shifting loyalties
After the accession of Edward V of England to the throne in 1483, Stanley sought to maintain the balance of power between the Duke of Gloucester and the king's maternal family, the Woodvilles. After King Richard seized power, he appointed Stanley to the Order of the Garter, replacing the late William Hastings, 1st Baron Hastings, who had been executed by the king. Stanley assisted the King in suppressing the Duke of Buckingham's failed rebellion, and he was rewarded with the title of Lord High Constable.

In 1485, however, Stanley's stepson Henry Tudor invaded England through Wales, intent on claiming the throne for himself. Stanley arranged for his forces to defect to Tudor when battle was met. Sure enough, at the Battle of Bosworth Field, King Richard lost his numerical advantage when Stanley's forces defected to Tudor's army, and it was Stanley's troops who would surround and mortally wound King Richard. King Henry demonstrated his gratitude by making Stanley Earl of Derby later that year. Stanley became a landed magnate of immense power, ruling the northwest of England unchallenged. He died in 1504.